Low-nickel ductile steel



y 1966 HAJlME- NAKAMURA 3,2

LbW-NICKEL DUGTILE STEEL Filed April 20, 1962 1O Sheets-Sheet 1 Trl5 and TrS Transition Tempera1ure,C

Precipitated Aluminum Nitride,%

INVENTOR. I HAJIME NAKAMURA BY M M ATTORNEY May 3, 1966 HAJIME NAKAMURA 3,249,425

0 LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet 2 0 0.02 0.04 0.06 0.08 0.|O Precipitated Aluminum Nitride,%

INVENTOR.

HAJIME NAKAMURA BYwiw A TTOR NE Y y 3, 1966 HAJIME NAKAMURA 3,249,426

LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet 3 T1 El.

' I 4O N.1Normalized Specimen No.4,QI Q.T.I Quench-und-Tempered I Commercial Steel:

c........o.|'r% 35 Ni.....'..2.36

I Specimen No. 4,N.

Commercial 20 V- Notch Chorpy impact Volue,kg-m/cm 1 7 I5 I I I Commercial 1 SleeLN lo I I -200 -ISO -l20 -8O -40 0 TemperolurefC INVENTOR. HAJlME NAKAMURA BY miw ATTORNEY y 1966 HAJIME NAKAMURA 3,249,426

LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 i 10 Sheets-Sheet 4 NJ Normalized Q.T.I Queneh-und-Tempered Commercial Steel:

C........O.|2/e Ni....... 3.70%

IN E 25 Q g, Specimen No.8,QJ3Y g 20 E Specimen NQQN. g E 3 l5 2 o 2 f 7 an 5 Co merclol Steel,Q.T. Commercial SteeLN, I

-2OO 460. -|2O -BO -40 O 40 Temperature, C

INVENTOR. HAJIME NAKAMURA BY M i ATTORNEY May 3, 1966 LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 l0 Sheets-Sheet 6 QT. Trs 1:: v

N.,TrS I Z Metallic Aluminum in Solid Solution,% INVENTOR.

HAJIME NAKAMURA M f M ATTOR NE Y May 3, 1966 v HAJIME NAKAMURA 3,249,426

LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet a Metallic Aluminum in Solid Solu1ion,%

INVENTOR. HAJIME NA AMURA BY Mi ATTORNEY y 3, 1966 HAJlME NAKAMURA 3,249,426

LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet '7 Specimen No.4.

930Cx1 hr. air-cooled Specimen No.4

930Cxi hr. ,oircooled INVENTOR. HAJIME NAKAMURA ATTORNEYS y 1966 HAJIME NAKAMURA 3,249,426

LOW-NICKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet 8 Specimen No.4

930 C x 1 hr. wcter-cooled 650Cx1.5hrs., air-cooled Specimen No.4

930 0 x1 hr. wafer-cooled 650Cx 1.5hrs. air-cooled INVENTOR. HAJIME NAKAMURA ATTORNEYS y 1966 HAJIME NAKAMURA ,4

LOW-NI CKEL DUCTILE STEEL Filed April 20, 1962 10 Sheets-Sheet 9 Specimen No.8

930Cx1 hr. air-cooled Specimen No.8

930Cx1 hr. air-cooled INVENTOR. HAJIME NAKAMURA ATTORNEYS y 1966 HAJIME NAKAMURA 3,249,426

LOW-NICKEL DUGTILE STEEL Filed April 20, 1962 10 Sheets-Sheet 10 Specimen No.8

930C xi hr., water-cooled 650Cx 1.5 hrs. air-cooled Specimen No.8

930Cx1 hr. water-cooled 650Cx1.5 hrs. ,oir-cooled INVENTOR. HAJIME NAKAMURA ATTORNEYS United States Patent 3,249,426 LOW-NICKEL DUCTILE STEEL Hajime Nakamura, Tokyo-t0, Japan, assignor to Ishikawajima-Harima Jukogyo Kabushiki Kaisha, Tokyo-t0,

invention in two different states of heat treatment, namely, of normalization and quench-and-tempering,

FIG. 2 is a similar drawing but of a 3.5% Ni grade steel of the present inventionin like state of heat treattransition temperature as well as the shear fracture transition temperature of a 2.5% Ni grade steel of the present Japan a corporation of Japan ment as above FIG. 3 is a drawing in which the transition temperature Cl ai 53 f g k ii gh if 2:3 1961 characteristics of a 2.5 Ni grade steel of the present 36/17 369 invention is compared with that of a commercially avail- 3 Claims 1 able steel of the same grade in two different states of heat treatment as above,

It has been known in the art that the addition of Ni is 4 is a Similar drawing but of a N1 [very ff ti i improving h 1 temperature toughsteel of the present invention and of a commercially availness or in eliminating the low temperature brittleness of able Same g d Steel in the 531116 State of heat treat a steel. Thus, in materials for services at a temperature Infill as above, below 40 1 Steels f 5% 35% i or FIG. 5 is a drawing to show the relation between the 9% grade are in the general use While, for ervices 15 fir-lb. transition temperature as Well as the shear fracat still lower temperatures, high-NiStee1s such a an ture transition temperature and the amount of metallic .tenitic stainless steel of 18-Cr, 8-Ni class are most comaluminum in Solid Solution for a given amount of P moniy cipitated aluminum nitride, of a 2.5 Ni grade steel of However if the amount f Ni is to b incl-eased i the present invention in two different states of heat treatorder to obtain a better serviceability at lower tempera- Infill as ahOVe, tures, the price of the steel tends to become so much 6 is a like drawing but of a Ni grade-filed hi h that i is of great importance today to meet a given of the present invention and of a commercially available service requirement i as little Ni as i l same grade steel in the same state of heat treatment as The present inventor is aware that, if a refinement and above, uniformity could be realized in the granular structure of FIGS- 7 and h electron photomlcl'oglams of a steels of plain carbon or low-alloy grade, the low tem- Ni grade stecl 111 a State of flofmalllahon at magnlfiperature ductility of said steels could be greatly im- Cations 0f and p l proved, and further, it has been found today, after many FIGS- 9 and 10 are electron Photomlcmgrams 0f the inquiries and tests, that the low temperature brittleness Same Steel but in the State of q h p at the of 21 Ni steel can also always be improved to an extent Same magnifications above, p lf, that is far greater than what is to be expected of the FIGS- 11 and 12 are electron Photomlcrogfams of a amount f Ni involved therein, by propel-Honing the 3.5% Ni grade steel of the present invention in a state amount of precipitated aluminum nitride with respect to of normahzatloh at magnifications of X and that of metallic aluminum in solid solution. 10 0O0X, lespectlvelyi It is an object of present invention to provide a steel G 13 and 14 are electron Photomlcfogfams 0f the which has an excellent ductility at low temperature for Same steel but in the State of quehCh-ahdtemP'eTihg at a price appreciably lower than that of an existing comthe same magnifications ashhove, P Y- mercially available steel with comparable composition. chelhlcal comlfoslhohs ffipfesfinlahva Thus, the present invention relates to low-Ni ductile steels 40 N1 grade and 35% N1 grade Steels used 111 the venous consisting of less than about 0.25% carbon, less than experiments are Summarized in Table It Will be Seen about 0.60% silicon, less than about 1.00% manganese, that specimens Nos. 1 to 5 represent the 2.5 Ni grade less than about 0.030% phosphorus, less than about steels and Nos. 6 to 10 represent the 3.5% Ni grade 0.030% sulphur, about 1.00 to 4.00% nickel, about 0.02 steels.

Table 1 Spec1- C Si Mn P S Ni N Total AlN Metallic men N0 A1 A1 to 0.12% precipitated aluminum nitride, and less than The changes in transition temperature in terms of 15 about 0.15% metallic aluminum in solid solution, the ft.-lb. level, the TrlS (which will be referred to simply as balance substantially all iron with incidental impurities, Tr15 hereinafter), versus various amounts of precipithe percentage being in weight percent, the steel having tated aluminum nitride, appearing in the above Table 1 a granular structure with grain size number larger than as AlN, is illustrated in FIG. 1 for 2.5 Ni grade steels No. 9 in terms of the ASTM Ferrite Grain Size Number. listed in Table 1. Another kind of transition tempera- In the following, the principle and scope of the prestures in terms of 50% crystalline or shear-fracture level, ent invention will be disclosed in detail, references being the TrS (which will simply be referred to as TrS hereinmade to drawings and photograms, where, after), of the same steels are also presented in the same FIG. 1 is a drawing to show the relation between the figure as a function of the amount of precipitated alumiamount of precipitated aluminum nitride and the 15 ft.-lb. num nitride. The specimens were heat treated prior to the transition temperature test in either one of two different ways, one involving heating to 930 C. for one hour foln3 lowed by cooling in air therefrom (this process of heat treatment will simply be referred as as normalization hereinafter), the other involving heating to 930 C. for one hour followed by rapid cooling in water, then reheating to 650 C. for 1.5 hours followed by cooling in air (which will simply be referred to as quench-and-tempering hereinafter).

An improvement or a downward trend is apparent in either of these transition temperatures of TrlS or TrS in that bothare shifted toward the lower temperature side as the amount of precipitated aluminum nitride is increased. It may be seen, however, that such is more evident in the case of normalization where the Tr15 is lowered from to -131 C. as the amount of precipitated aluminum nitride is increased from 0.0028% to about 0.073%. The improvement in the TrS, on the other hand, is from +40 to -78 C. in the same range of changes in the amount of precipitated aluminum nitride. Such reduction of transition temperature is not so great in the case of quench-and-tempering as in the case of normalization. Yet a definite improvement is to be observed in the same figure, in which the Tr is lowered from 137 to -163 C. for a change in the amount of precipitated aluminum nitride of 0.0028% to about 0.073%, while that in the TrS is from -110 to 118 C. for the same range of precipitated aluminum nitride.

A similar trend was observed also in the change of Tr15 and TrS of 3.5% Ni grade steels as may be seen in FIG. 2. Here again, the influence of precipitated aluminum nitride is greater for normalized material in that TrlS is lowered from +25 to -l06 C. corresponding to the increasing amount of precipitated aluminum nitride from 0.0028% to about 0.071%, while that in TrS is from +58 to 54 C. over the same range of changes in the amount of precipitated aluminum nitride. The improvement in transition temperatures for quench-and-tempered material is again less, yet a substantial gain is to be noted. Namely, the Tr15 is lowered from 102 to 156 C. for the same increment in the amount of precipitated aluminum nitride as above with the lowest or best value of -l62 C. at about 0.05% of the latter component, While that in the TrS was 71 to 114 C. over the same range of the latter.

The superiority of low temperature ductility of steels due to the present invention are well demonstrated by FIGS. 3 and 4, in which the transition temperature characteristics of a 2.5% Ni grade steel and a 3.5% Ni grade steel of the present invention are comparatively shown, respectively, with those of commercially available steels of equivalent compositions, respectively, in two different states of heat treatment of normalization and quenchand-tempering. It is to be seen that at any given temperature, the impact strength, as determined by Charpy V-notch test method, of a steel due to the present invention is greater than that of a commercial steel of comparable composition. In terms of the Tr15 of the 2.5 Ni grade steels, for example, that of a steel of the present invention in a state of quench-and-tempering is about --165 C., while that of commercial steel in the same state is about 100 C., representing a net improvement of about 65 C. For the same steels but in the normalized state, the T115 is about -140 C. for the steel of the present invention, and about -80 C. for the commercial steel, the gain being about 60 C. in this case.

The precipitated aluminum nitride which helps to render the steels of the present invention remarkably impact resistant is in a dimension of such a small magnitude that it cannot be detected save by electron microscope. FIGS. 7, 8 and 9, 10 illustrate the ferrite grains of a 2.5% Ni grade steel (specimen No. 4) together with the precipitated aluminum nitride as observed under an electron microscope at magnifications of 4,000 or 10,000X, the stock being in state of either as normalized or as quenchand-ternpered, respectively. In the photogram, those rectangular shaped dark bars are the precipitated aluminum nitride. It will be seen in the photograms that the ferrite grain size is smaller in the quench-and-tempered material (FIGS. 9 and 10) than that in the normalized state (FIGS. 7 and 8).

Similar observations were obtained for a 3.5% Ni grade steel (specimen No. 8) as demonstrated by like electron photomicrograms shown in FIGS. 11,12 and 13, 14, showing dark rectangular objects that are the precipitated aluminum nitride. It is to be noted here again that the ferrite grain size is finer in the quench-and-tempered material (FIGS. 13 and 14) than that in the normalized condition (FIGS. 11 and 12).

Though the effectiveness of such precipitated aluminum nitride is thus evident, it should be noted that if the amount is not sufficient, not only can such excellent low temperature ductility not be expected, but an actual deterioration of that property may occur. This circumstance may be seen by comparatively reviewing FIGS. 3 and 4 with FIGS. 1 and 2, respectively. It will be seen, for instance, that the Tr15 value of a commercially available 2.5% Ni grade steel containing 0.17% C., 2.36% Ni and of the order of 0.006% nitrogen,,is about C. in a state of normalization (FIG. 3), but that of the specimen No. 1 that contains 0.12% C., 2.65% Ni, 0.023% nitrogen but only 0.0028% precipitated alumin um nitride (Table 1) is merely -10 C. (FIG. 1), although those in quench-andtempered state are about C. for the former and l37 C. for the latter (FIGS. 3 and 1, respectively). A similar relation is also observable between a commercial 3.5% Ni grade steel containing 0.12% C., 3.70% Ni and of the order of 0.006 nitrogen, and the specimen No. 6 that contains 0.16% C., 3.71% Ni, 0.023% nitrogen but merely 0.0028% precipitated aluminum nitride, in that the Tr15 of the former in normalized condition is about -100 C. (FIG. 4) and that of the latter in the same state is +25 C. (FIG. 2), the Tr15s in the quench-andtempered state are C. forthe former (FIG. 4)

.and 102 C. for thelatter (FIG. 2). The harmful effect of nitrogen left free in steel is thus evident here also, as is known in the art.

As has been described heretofore, such harmful effect of nitrogen contained in steel can not only be eliminated by combining it with aluminum into a form of aluminum nitride, but can also actually be made useful in providing the steel with additional increase in the low temperature ductility by precipitating out this aluminum nitride dispersedly and uniformly as seen in FIGS. 1 and 2. However, there exists a certain natural law and an empirical rule to govern such amount of aluminum nitride to be contained in steel matrix.

Firstly, the upper limit is dictated by the natural solubility of nitrogen in molten steel to be about 0.035% maximum, which corresponds to about 0.12% in terms of aluminum nitride. The lower limit is determined by the value of Tr15 then to be expected which is customarily put at the 40 to -50 C. range by various industrial specifications such as, for instance, the Standard due to the American Society for Testing Material (ASTM A 300-58). Such TrlS value is to be obtained from steels due to the present invention at an amount of precipitated aluminum nitride of about 0.02% as may be judged by FIGS. 1 and 2.

Such very favorable effect of precipitated aluminum nitride, however, is not to be expected unless all, or at least the substantial portion of, the nitrogen is caught and combined by aluminum, as described in the preceding par-agraphs in connection with the harmful effect of free nitrogen. An excessive addition of aluminum for a purpose of eliminating all the nitrogen to trace, on the other hand, will again result in the rise of the transition temperature, for excessive aluminum then contained in the matrix as in solid solution gives rise to coarsening of the ferrite grains. The circumstance described above may be well understood by FIGS. 5 and 6, in which the changes in the TrlS values with increasing amount of metallic aluminum in solid solution are illustrated for 2.5 Ni and 3.5% Ni grade steels, respectively, in two states of heat treatment of normalization and quench-and-tempering. In these cases, the amount of precipitated aluminum nitride was kept constant at about 0.07% level. It will be seen that for Tr15s of 80 C. and 120 C. for 2.5% Ni grade in states of normalization and quench-and-tempering, respectively, given in accordance with the current practices of specifications, the corresponding values for metallic aluminum in solid solution are 0.13% and 0.15%, respectively. At the 3.5% Ni grade, the normally specified TrlS is -100 C. for normalized material and l40 C. for quench-and-tempered stock, and the corresponding amount of precipitated aluminum nitride is 0.014% for the former case and about 0.2% for the latter. Therefore, a limiting value of about 0.15% is indicated for metallic aluminum in solid solution.

Although a number of examples have been described heretofore with respect to grades of 2.5% Ni and 3.5% Ni only, it should be evident that the principle of the present invention is not to be restricted to those specific examples, but to be considered as applicable to all and general so-called low-nickel grade containing less than about 0.25% carbon, less than about 0.6% silicon, less than about 1.00% manganese, less than about 0.030% phosphorus and sulphur each, about 1.00 to 4.00% nickel. It should be evident further that by prescribing and proportioning the respective amounts of precipitated aluminum nitride and metallic aluminum in solid solution to about 0.02 to 0.12% and less than 0.15%, respectively, any and all the so-called low-nickel grade steels can be remarkably improved with regard to low temperature due- 6 tility, and the grain size thereof can be greatly refined so as to be larger than No. 9 in terms of the ASTM Ferrite Grain Size Number.

I claim:

1. A structural steel consisting of less than 0.25 carbon, less than 0.60% silicon, less than 1.00% manganese, less than 0.030% phosphorus and sulphur each, 1.00 to 4.00% nickel, 0.02 to 0.12% precipitated aluminum nitride, less than 0.15% metallic aluminum in solid solution, the balance substantially all iron With incidental impurities, the steel having a granular structure with grain size number larger than No. 9 in terms of the ASTM Ferrite Grain Size Number.

2. A method of heat treatment for the steel according to claim 1, consisting of rapidly cooling in air or water, the starting temperature being a temperature above the A transformation temperature and within the temperature range for fast precipitation of aluminum nitride.

3. The method according to claim 2 wherein the step of tempering at a temperature between C. and A transformation temperature follows the rapid cooling.

References Cited by the Examiner Alloys of Iron and Nickel, vol. 1, pages 398, and 399. Edited by Marsh, published in 1939 by the McGraW-Hill Book Company, New York.

Bureau of Standards Journal of Research, vol. 48, pp. 193 to 199.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. A STRUCTURAL STEEL CONSISTING OF LESS THAN 0.25% CARBON, LESS THAN 0.60% SILICON, LESS THAN 1.00% MANGANESE, LESS THAN 0.030% PHOSPHORUS AND SULPHUR EACH, 1.00 TO 4.00% NICKEL, 0.02 TO 0.12% PRECIPITATED ALUMINUM NITRIDE, LESS THAN 0.15% METALLIC ALUMINUM IN SOLID SOLUTION, THE BALANCE SUBSTANTIALLY ALL IRON WITH ICIDENTAL IMPURITIES, THE STEEL HAVING A GRANULAR STRUCTURE WITH GRAIN SIZE NUMBER LARGER THAN NO. 9 IN TERMS OF THE ASTM FERRITE GRAIN SIZE NUMBER. 